Method and aritcle for inhibiting gaseous permeation and corrosion of material

ABSTRACT

A method and apparatus for inhibiting gaseous permeation and corrosion of materials is disclosed. Gaseous permeation and corrosion of materials is inhibited by the provision of a coating of gold and tantalum over the surface of the article to be protected. In a preferred method of applying the coating, the gold and tantalum are co-deposited by sputtering to form the coating. The gold and tantalum coating is preferably protected by an organic coating to form a barrier to prevent contact of the gold and tantalum coating with corrosion-producing materials. The gold and tantalum coating reduces hydrogen permeation of coated materials by factors of several thousand compared to a reduction factor of approximately five times for a pure gold coating of comparable thickness.

United States Patent Seldon et al.

METHOD AND ARITCLE FOR INHIBITING GASEOUS PERMEATION AND CORROSION OFMATERIAL Inventors: Martin M. Seld0n; Leo J. Garbini, both of Sunnyvale,Calif.

Assignee: Varian Associates, Palo Alto, Calif.

Filed: Nov. 30, 1970 Appl. No.: 93,902

References Cited UNITED STATES PATENTS Knippenberg et al. ..75/165Spence ...29/183.5 Laegreid et al ..204/l92 Apr. 3, 1973.Att0rneyStanley Z. Cole and Vincent W. Cleary [57] ABSTRACT A methodand apparatus for inhibiting gaseous permeation and corrosion ofmaterials is disclosed. Gaseous permeation and corrosion of materials isinhibited by the provision of a coating of gold and tantalum over thesurface of the article to be protected. in a preferred method ofapplying the coating, the gold and tantalum are co-deposited bysputtering to form the coating. The gold and tantalum coating ispreferably protected by an organic coating to form a barrier to preventcontact of the gold and tantalum coating with corrosion-producingmaterials. The gold and tantalum coating reduces hydrogen permeation ofcoated materials by factors of several thousand compared to a reductionfactor of approximately five times for a pure gold coating of comparablethickness.

5 Claims, No Drawings METHOD AND ARITCLE FOR INHIBITING GASEOUSPERMEATION AND CORROSION OF MATERIAL GOVERNMENT CONTRACT The inventionherein described was made in the course of or under a contract with theUS. Air Force.

DESCRIPTION OF THE PRIOR ART Heretofore, articles and devices have beencoated with gold to reduce permeation of the coated article or device byhydrogen. Samples of prior art patents directed to use of a gold coatingare: US. Pat. No. 3,239,300 issued Mar. 8, 1966 and US. Pat. No.3,284,175 issued Nov. 8, l966. The former patent is directed to a goldcoating on the metallic envelope portion of an electron tube, whereasthe second patent is directed to a gold coating on high strength steelarticles to prevent embrittlement and weakening of the steel article byhydrogen permeation. It has been found that a gold coating reduceshydrogen permeation of the underlying material by a factor ofapproximately five. It is desirable to obtain an improved coating whichgreatly reduces the gaseous permeation of the coated member by factorsgreatly in excess of five.

SUMMARY OF THE PRESENTINVENTION The principal object of the presentinvention is the provision of improved method and apparatus forinhibiting gaseous permeation and corrosion of materials.

One feature of the present invention is the provision of agaseous-permeation inhibiting coating consisting of gold and tantalum. I

In another feature of the present invention, a gold and tantalum coatingfor inhibiting gaseous permeation of an underlying article is protectedby an overlaying coating of organic resin selected from the classconsisting of fluorocarbon resin, silicone resin, epoxy resin,silicone-alkyd co-resins, and epoxy co-resins with phenolic, urea ormelamine.

In another feature of the present invention, the material which formsthe gaseous permeation-inhibiting coating is applied by co-deposition ofgold and tantalum.

In another feature of the present invention, the gold and tantalumgaseous-permeation-inhibiting layer is formed by co-sputtering gold andtantalum onto the article.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been discovered that acoating of gold and tantalum over a material greatly inhibits permeationof the coated surface by gases, such as hydrogen. For example, a coldrolled steel membrane which is 0.002 inch thick and coated with a layerof co-sputtered gold and tantalum to a thickness of approximately 3microns with an overlaying protecting coating of silicone resin reduceshydrogen permeation of the membrane by a factor of several thousand ascompared with a reduction of only six times for a silicone resin coatedmembrane and a reduction of only five times for a gold coated membrane.

7 cent, for improved performance. The cold rolled steel membrane whichwas coated was analyzed and found to contain constituents other thaniron in the following parts per million: carbon, 800 parts per million;manganese, 400 parts per million; nickel, 170 parts per million;chromium, 80 parts per million; and copper, 40 parts per million. Thepermeation tests were conducted at 100C with hydrogen gas supplied tothe surface of the membrane at approximately 1 atmosphere pressure. Avacuum was pulled on the opposite side of the membrane. The coatedmembrane was baked in vacuum at 400C for 18 hours prior to the gaspermeation test.

It is believed that the manganese and iron diffused to the surface ofthe membrane through the gold and tantalum coating during the bakingprocess. A black coating was found on the high vacuum side of themembrane and it is believed that this occured due to diffusion of themanganese out of the iron membrane. The manganese coating had no effecton hydrogen permeation.

It is believed that the greatly reduced gaseous permeation rates throughthe gold and tantalum coating is due to the fact that the gold providesa smooth and continuous coating that tends to fill in surfacediscontinuities in the material being coated and that hydrogen isactively absorbed by the tantalum in the coating and results in hydrogenbeing held on the surface of the coating in some relatively stable form,such as one of the tantalum hydrides or other stable compounds. Thesilicone resin overcoat serves to inhibit exposure of the gold andtantalum coating to moisture or other corrosion-producing materials.

The gold and tantalum coating is preferably formed by co-sputteringtantalum and gold onto the surface of the article to be protected. Aparticularly useful apparatus for sputtering gold and tantalum onto thematerial to be coated is low energy sputtering equipment manufactured byConsolidated Vacuum Corporation in which three elements are employed forsputtering in a plasma discharge generally formed in an inert gas suchas argon, krypton, or Xenon at a reduced pressure, as of l X 10' torr.Such an apparatus is described in an article titled, Some PracticalConsiderations In The Use Of Low Energy Sputtering appearing in theDec., 1965 issue of SCP and Solid State Technology, pages 30-33.

The gold and tantalum coating is preferably protected by an organiccoating selected from the group consisting of fluorocarbon resins,silicone resins, epoxy resins, silicone-alkyd co-resins, and epoxyco-resins with phenolic, urea or melamine. A particularly suitableorganic protective material comprises silicone resin marketed under thename Vac-Seal by Space Environment Laboratories of Boulder, Colorado,such material being available in spray can form for spraying theprotective silicone coating onto the gold and tantalum layer.

Silicone resins are well-known for their ability to repel water and toresist heat. Many of these resins will withstand temperatures in excessof 500F. The following pure silicone resins may be utilized for theprotective coating over the gold and tantalum layer: Dow- Corning, 804,805, 806 A, 808, and 847; General Electric SRlll, SR112, SR53, SR82,SR119, SR124 and SR125; and Union Carbides Ucarsil 104. For thin films(less than 1 mil), SR53 and Ucarsil104 are preferred. For thicker films(1-3 mils) DC 808, GE SRll 1, SR112, SR119, and SR125 are preferred.

There are many epoxy resins available with not too widely differentproperties. They are better than silicone resins in hardness andcorrosion resistance, but are not as good in heat resistance andmoisture resistance. A typical epoxy resin is Shell Epon 1001.

The gaseous-permeation-inhibiting gold and tantalum coating with theprotective organic coating is particularly useful for coating of vacuumenclosures of electron tubes, such as klystrons, traveling wave tubes,magnetrons and the like. These coatings prevent hydrogen gas frompermeating the envelope and producing a number of free gas ions withinthe evacuated device. The coating is also useful on space vehicles,cryogenic and undersea pressure vessels, and vehicle wall constructionsas well as in special structural steel applications.

The coatings of the present invention are also applicable to otherdevices for protection against corrosion. When the gold and tantalumcoating is used in conjunction with a coating of organic resin, such assilicone resin, the formation of corrosion products is inhibited.Corrosion products or products formed by gas and ion collisions at thesurface of the coated article are prevented from passing through thematerial by the coating. Under typical atmospheric conditions, ion andgas collisions at the surface of an object result in dissociation of thegases which are present. One of the results of this surface activity isthe generation of atomic hydrogen that freely enters the surface layersof the object and then moves through the material. Dependent upon thecondition of the material, hydrogen, in the atomic state, can recombineat the inside surface in a variety of manners to generate deleteriouscompounds, such as molecular hydrogen, methane, water, etc. Othercompounds which are not products of hydrogen reaction, but ofsubstitution reaction, include carbon monoxide and carbon dioxide. Useof the coatings of the present invention inhibit these phenomena to aconsiderable degree.

Permeation of hydrogen into metals can have a variety of undesirableeffects on the characteristics of the metal. Such undesiredcharacteristics include embrittlement and weakening of the metal such asthose discussed in the aforecited US. Pat. No. 3,284,175.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description shall be interpreted as illustrativeand not in a limiting sense.

What is claimed is: 1. In a method for inhibiting hydrogen permeation ofthe surface of an article, the steps of:

simultaneously sputtering gold and tantalum particles onto said surfaceto form a gold and tantalum coating on said surface, and

heating the coating at about 400 C for about 18 hours.

2. The method of claim 1 in which said gold and tantalum coating isabout 3 microns thick.

3. An article having an outer surface which is resistant to hydrogenpermeation, comprising:

a first member having a selected surface area, and

a gold and tantalum coating on said selected surface area to form saidouter surface of said article, said coating being formed by the stepsof:

simultaneously sputtering gold and tantalum particles onto said surfacearea to form said gold and tantalum coating on said surface area, and

heating the coating at about 400 C for about 18 hours.

4. The article of claim 3 in which said gold and tantalum coating isabout 3 microns thick.

5. An electron tube comprising an anode; a cathode; evacuated envelopemeans for enclosing said anode and cathode, said envelope means having aselected surface area; and a gold and tantalum coating on said surfacearea for inhibiting hydrogen permeation of said evacuated enclosure,said gold and tantalum coating being formed by the steps of:

simultaneously sputtering gold and tantalum particles onto said surfacearea to form said gold and tantalum coating on said surface area, and

heating the coating at about 400 C for about 18 hours.

2. The method of claim 1 in which said gold and tantalum coating isabout 3 microns thick.
 3. An article having an outer surface which isresistant to hydrogen permeation, comprising: a first member having aselected surface area, and a gold and tantalum coating on said selectedsurface area to form said outer surface of said article, said coatingbeing formed by the steps of: simultaneously sputtering gold andtantalum particles onto said surface area to form said gold and tantalumcoating on said surface area, and heating the coating at about 400* Cfor about 18 hours.
 4. The article of claim 3 in which said gold andtantalum coating is about 3 microns thick.
 5. An electron tubecomprising an anode; a cathode; evacuated envelope means for enclosingsaid anode and cathode, said envelope means having a selected surfacearea; and a gold and tantalum coating on said surface area forinhibiting hydrogen permeation of said evacuated enclosure, said goldand tantalum coating being formed by the steps of: simultaneouslysputtering gold and tantalum particles onto said surface area to formsaid gold and tantalum coating on said surface area, and heating thecoating at about 400* C for about 18 hours.